1. Field of the Invention
The present invention concerns an improved shell-and-tube apparatus having an intemediate tube plate, specifically, a shell-and-tube reactor. The invention is particularly useful when applied to the apparatus for producing (meth)acrylic acid by catalytic gas-phase oxidation of propylene or butenes.
2. State of the Art
There are many processes in which quenching of reaction gas is neccessary, for example, the process for producing acrylic acid by catalytic gas-phase oxidation of propylene. As the reaction apparatus for practicing such a process, a shell-and-tube reactor having an intermediate tube plate is the most suitable.
The shell-and-tube reactor having an intemediate tube plate is a reactor of the structure having, in addition to an intermediate tube plate which sections the reactor into two compartments, a high temperature compartment and a low temperature compartment, plural reaction tubes penetrating the tube plate, thereby to heat or cool the reactant substances passing through the reaction tubes with a heat transfer medium surrounding the tubes.
In reference to the above mentioned process for producing acrylic acid from propylene, the material propylene gas is introduced with air and an inert gas (nitrogen or steam) into the reaction tubes in the high temperature side (reaction zone), and is converted to acrolain and acrylic acid by a solid catalyst packed in the reaction tubes.
After completion of the reaction, the gas is immediately cooled in the same reaction tube at the low temperature side (quenching zone) without being transferred to another vessel for the purpose of preventing undesirable side reactions. In an alternative embodiment, two kinds of catalysts are packed in a series in each reaction tube, and propylene is converted partly into acrolain and partly into acrylic acid by the first catalyst, and then, the acrolain is further converted to acrylic acid by the second catalyst so that the yield of the acrylic acid may increase. In the latter embodiment, it is necessary to carry out the respective reactions at the most suitable temperatures, and therefore, the reaction gas is rapidly cooled or heated at different positions of the same reacion tubes.
In order to quench the reaction gas in the former process, it is essential to keep the temperature difference between the reaction zone and the quenching zone of the reaction vessel. This causes heat transfer from the reaction zone by way of the intermediate tube plate to the quenching zone, and makes it sometimes difficult to keep the quenching zone at a determined temperature. The large temperature difference causes large thermal stress in the intermediate tube plate and the body wall to which the intermediate tube plate is fitted. The same problem resides more or less in regard to the rapid heating and quenching in the latter process.
The simplest way of fitting up the intermediate tube plate to the reaction vessel is, as shown in FIG. 5A and FIG. 5B, to weld the intermediate tube plate 2 as it is to the body wall 1.
In this case of direct welding the intermediate tube plate to the body wall, however, if the temperature difference between the two compartments of both sides of the plate is very large, the temperature gradient at the body wall near the intermediate tube plate is so steep that strong thermal stress occurs in the body wall. If the temperature difference rises above a certain level, the thermal stress exceeds the allowable stress of the body wall and the vessel may be destroyed. This is a particularly serious problem for vessels having thick intermediate tube plates.
Therefore, the reaction vessels are often constructed into two separate parts of different temperatures, i.e., the compartment of the reaction zone and the compartment of the quenching zone, and then, to connect the parts at the flanges thereof using a gasket therebetween; in other words, to form a shell and tube reaction vessel having two intermediate tube plates.
In this type of the reaction vessel there is an improvement that the thermal stress in the tube plates is not so large due to the lack of the intermediate tube plate at which a significant temperature difference is posed. However, there is a space between the reaction zone and the quenching zone, and therefore, the rate of cooling cannot exceed a certain limit. Also, the cost of manufacturing the apparatus becomes higher because the two tube plates are used.